1. Field of the Invention
The present invention relates to a contrast control circuit for a display apparatus, and more particularly, to a contrast control circuit for a display apparatus, which receives a first voltage of positive polarity adjusted by a user, and outputs a second voltage of negative polarity for controlling the contrast of a liquid crystal display panel.
2. Description of the Related Art
Referring to FIG. 1, a liquid crystal display apparatus 1, which is one of general display apparatuses, includes a liquid crystal display panel 11 and a driving apparatus thereof.
In the liquid crystal display panel 11, signal electrode lines SL1, SL2, . . . , SLm are parallel arranged on cells of liquid crystal (LC), and scan electrode lines CL1, CL2, . . . , CLn are parallel arranged under the LC cells to be orthogonal to the signal electrode lines SL1, SL2, . . . , SLm. The scan electrode lines CL1, CL2, . . . , CLn and the signal electrode lines SL1, SL2, . . . , SLm are made of a transparent conductive material, for example, ITO (indium tin oxide).
The driving apparatus includes a controller 14, a segment driver 12, a modulation-signal generator 141 and a common driver 13. The controller 14 processes a video signal Sc from a host computer, e.g., a notebook computer, and generates a data signal DATA, a shift clock signal SCK, a frame signal FLM and a latch clock signal LCK.
The segment driver 12 makes the input data signal DATA stand by in the respective signal electrode lines SL1, SL2, . . . , SLm according to the shift clock signal SCK. Also, a signal voltage corresponding to the data signal DATA standing by according to the latch clock signal, is applied to the respective signal electrode lines SL1, SL2, . . . , SLm. Here, a circuit of generating bias voltages VBS supplied to the segment driver 12 is shown in FIG. 2.
The frame signal FLM indicates starting of a frame. The modulation-signal generator 141 divides the frequency of the latch clock signal LCK and generates a modulation signal. The generated modulation signal controls the polarity of voltages output from the segment driver 12 and the common driver 13.
The common driver sequentially applies the corresponding scan voltages to the respective scan electrode lines CL1, CL2, . . . , CLn by controlling the latch clock signal LCK, the frame signal FLM and the modulation signal. Accordingly, the arrangement of liquid crystal (LC) of a to-be-displayed cell changes and light is transmitted or blocked. Here, a circuit of generating bias voltages VBC supplied to the common driver 12 is shown in FIG. 2.
Referring to FIG. 2, the circuit of generating bias voltages applied to the common driver (13 of FIG. 1) and the segment driver (12 of FIG. 2), includes a contrast control circuit 151 and a bias-voltage generator 15. The contrast control circuit 151 receives a first voltage of positive polarity adjusted by a user and outputs a second voltage VOUT of negative polarity for controlling the contrast of a liquid crystal display panel. The bias-voltage generator 15 produces bias voltages VBC and VBS applied to the common driver 13 and the segment driver 12 according to the output voltage VOUT of the contrast control circuit 151.
FIG. 3 is a detailed circuit diagram of the contrast control circuit 151 of the conventional liquid crystal display apparatus.
Referring to FIG. 3, the positive voltage applied to an input port 2 of an operation amplifier (OP-AMP) is determined by a positive supply voltage VCC and eighth, ninth and tenth resistors R8, R9 and R10. Reference symbol C represents a capacitor for removing radio frequency noise. The resistance of the ninth resistor R9 varies by user""s manipulation. For example, the positive voltage applied to the input port 2 of the OP-AMP varies in the range from 0 to 3 V.
The OP-AMP operable by positive and negative supply voltages VCC and VEE, outputs a negative voltage VOUT, the absolute value of which is proportional to the positive voltage applied to the input port 2. The absolute value of the output voltage VOUT is proportional to the combined resistance value of eleventh, twelfth and thirteenth resistors R11, R12 and R13. The negative voltage VOUT output from the OP-AMP is applied to the bias-voltage generator (15 of FIG. 3) as a contrast controlling voltage. The rated contrast controlling voltage VOUT applied to the bias-voltage generator 15 ranges from xe2x88x928 to xe2x88x9211 V, for example.
According to the aforementioned contrast control circuit for the conventional display apparatus, since an OP-AMP which is commercially available at high cost is necessarily used, the manufacturing cost increases. Also, circuit stability cannot be improved and power consumption increases.
To solve the above problems, it is an object of the present invention to provide a contrast control circuit for a display apparatus, which can reduce the manufacturing cost and power consumption and can improve circuit stability.
Accordingly, to achieve the above object, there is provided a contrast control circuit for a display apparatus, which receives a first voltage of positive polarity adjusted by a user, and outputs a second voltage of negative polarity for controlling the contrast of a display panel. The contrast control circuit includes third and fourth resistors, a first transistor, a second transistor, serially connected resistors and a Darlington circuit. The third and fourth resistors each have one end connected to the terminals of a positive supply voltage. The first transistor has its emitter connected to the other end of the third resistor and its base to which the first voltage is input. The second transistor has its emitter connected to the other end of the third resistor, its base connected to the other end of the fourth resistor, and its collector connected to a terminal of a negative supply voltage. The serially connected resistors have one end connected to the base of the second transistor and the other end connected to the output terminal of the second volt age. The Darlington circuit, constituents of which are connected between each of the output terminal of the second voltage, the collector of the first transistor and the terminal of the negative supply voltage, operates such that the absolute value of the second voltage is inversely proportional to the absolute value of the first voltage.
In the contrast control circuit for a display apparatus according to the present invention, the voltage of the base terminal of the second transistor is equal to the first voltage and the absolute value of the second voltage is inversely proportional to the absolute value of the first voltage. Accordingly, the second voltage can be generated by the first voltage, without using an operation amplifier which is commercially available at high cost. Thus, the manufacturing cost and power consumption can be reduced, and the circuit stability can be improved.
Preferably, at least one of the fourth resistor and the serially connected resistors is a thermistor. Accordingly, the circuit stability depending on temperature can be more effectively improved.